1. Field of the Invention
The present invention relates generally to a device for providing duplex hands-free communication and more particularly, to providing duplex communication as a speaker telephone.
2. Description of the Prior Art
Prior art speaker phones for hands-free communication typically use fast switching circuitry to alternate between broadcasting and receiving. If this switching did not take place an annoying feedback squeal would occur. The switching, however, can be annoying in itself because the circuitry compares the strength of the broadcast and receive signals and allows the strongest signal to be transmitted. This means the parties which are communicating must take turns talking which can be awkward.
To avoid the need for switching while also preventing feedback squeal, other approaches employing a speaker and one or two microphones have been tried using various cancellation techniques. These attempts have thus far been unsatisfactory because of phase changes occurring between cancellation signals when connected to different transmission paths, or because return losses are not great enough to permit a satisfactory speaker acoustical level without inducing loop oscillations.
Another approach to duplex systems has been to use an echo cancellation scheme wherein the system generates an estimate of the echo and subtracts the estimate from the signal containing the echo. The estimate of the echo, however, is not always accurate and under certain conditions a feedback squeal may still be generated.